Abstract The external anal sphincter (EAS) is essential for maintaining fecal continence, which affects 6% to 19% of elderly individuals aged 65 years and older living in the community. Aging significantly affects anorectal function both neurogenically and myogenically. However, there is an unmet need of advanced and less- invasive approaches to quantitatively and objectively evaluate the neuromuscular function of the EAS. Anorectal imaging techniques with ultrasound or magnetic resonance imaging provide useful information in detecting anatomical defects in the EAS, but they are not capable of detecting neurogenic injuries. Intramuscular electromyography (EMG) is the only technique available for documenting neurogenic sphincter injury, but its application is limited by its highly invasive nature, poor repeatability and inability to describe global innervation. Pudendal nerve terminal motor latency has several limitations including the fact that a normal latency time does not exclude neuropathy and its clinical significance remains controversial. Anorectal manometry measures pressures of the anal sphincter muscles, sensation in the rectum, and neural reflexes that are necessary for normal bowel movements. However, normal ranges of various parameters measured are highly variable and poorly reproducible. The goal of this project is to develop a minimally invasive EAS innervation pattern characterization technique using the most recent advances in high-density (HD) surface EMG recording and signal processing techniques, to quantitatively and objectively assess the neuromuscular function of the EAS, 1) at rest when only tonic activities are present, 2) during voluntary contraction when motor units are selectively recruited, and 3) under supramaximal pudendal stimulation when all motor units are simultaneously activated, as well as to investigate specific alterations of the EAS innervation associated with aging. This research represents the first effort to quantitatively and objectively characterize the innervation patterns of the EAS under three different physiological conditions. The innervation patterns of the EAS characterized at rest, during voluntary contraction and under supramaximal pudendal nerve stimulation provide specific EAS function information associated with the resting tone, voluntary activation and global innervation of the EAS respectively. Aging effects on specific changes of the EAS innervation under these three conditions will be investigated. The proposed EAS innervation pattern characterization technique will serve as a novel phenotyping and predictive tool for anorectal dysfunctions caused by neurogenic weaknesses of the EAS. The results will advance our understanding of the pathophysiology of anorectal neuromuscular dysfunctions associated with aging, will also have clinical significance with regard to future therapeutic ?choice? of biofeedback or surgical intervention.